1. Field of the Invention
The present invention is broadly concerned with improved, prefabricated strain gage apparatus for sensing surface conditions and strain in test bodies. More particularly, it is concerned with such strain gage devices which are especially designed to facilitate accurate alignment and attachment of relatively delicate electrical gage components onto a test surface while substantially lessening gage installation times and the incidence of gage destruction during such installations.
2. Description of the Prior Art
In the design, development and testing of many structural components such as airplane wings or the like, it is desirable to understand the deflection and strain which the component may undergo during use. In order to measure these forces, electrical resistance-type strain gages have long been employed, and are generally capable of providing accurate information, so long as they are properly and accurately applied to a test surface. Generally speaking, such strain gages are very delicate devices, formed with metallic foil grids having elongated, laterally spaced, series-connected grid elements and enlarged terminal regions. They are typically used in conjunction with a pair of spaced conductive metal tabs to which external leads may be attached.
In general, such strain gage and tab assemblies are individually packaged in small envelopes and sold in lots. In order to attach a strain gage/tab assembly to a test surface, a relatively complex series of time-consuming steps must generally be followed. First, the test surface is thoroughly cleaned and degreased. Then, tweezers are used to remove the gage and tab components from an envelope, and these are placed on a glass plate or gage box surface. A stretch of cellophane adhesive tape is then applied over the gage and tab assembly, taking care to center the gage on the tape. The tape is then carefully lifted at a shallow angle, bringing the gage and tab assembly up with the tape.
In the next step, the tape is applied onto the desired test surface, with an attempt being made to properly align the gage grid with the precise point on the test surface under consideration. As necessary, the tape is lifted and realigned until the most accurate possible orientation of the gage is achieved. At this point, one end of the tape is lifted from the test surface until the attached gage and tab assembly is free of the test surface; the free end of the tape is then tucked under so that the gage and tabs are supported on the tape above the test surface. Catalyst is then applied to the exposed gage and tab surfaces, while complementary adhesive is applied to the test surface along the juncture of the tape.
In the final attachment steps, the free end of tape is rotated back towards the test surface and, while holding the tape slightly taut, a wiping stroke is made over the outer surface of the tape. This serves to bring together the adhesive and catalyst and firmly bond the gage and tab to the test surface; this bonding is completed by thumb pressure over the gage and tab regions of the tape. Finally, the cellophane tape is pulled directly back over itself to slowly and steadily peel the tape off the test surface and bonded gage/tab assembly.
As can be appreciated, this installation procedure is time-consuming and requires considerable practice and skill to master. It is moreover very easy to improperly attach and/or destroy the delicate and expensive strain gage components during the course of an attempted installation. Indeed, it is estimated that as many as 20-30% of expensive strain gages are rendered useless by improper installation.
There is accordingly a real and unsatisfied need in the art for an improved strain gage assembly and installation method which overcomes the problems outlined above and materially decreases gage installation time while minimizing gage destruction owing to improper attachment techniques.